Arc emission spectrometry

Electrical arc sources for atomic emission spectrometry use currents of between 5 and 30 A, and the burning voltage between two electrodes of different polarity in the case of an arc gap of a few mm then ranges between 20 and 60 V. DC arcs can be ignited by bringing the electrodes together, which causes some ohmic heat- 

The plasma temperature in the carbon arc is of the order of 6000 K and it can be assumed to be in local thermal equilibrium. According to the temperatures obtained, it could be expected that in arc emission spectrometry mainly the atom lines will be the most sensitive lines, when considering their norm temperatures. Arcs are usually used for survey trace analysis but also for the analysis of pure substances when the highest power of detection is required. However, they may be hampered by poor precision (RSDs of 30% and higher). 

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Qualitative. The classic method for the quaUtative determination of silver ia solution is precipitation as silver chloride with dilute nitric acid and chloride ion. The silver chloride can be differentiated from lead or mercurous chlorides, which also may precipitate, by the fact that lead chloride is soluble ia hot water but not ia ammonium hydroxide, whereas mercurous chloride turns black ia ammonium hydroxide. Silver chloride dissolves ia ammonium hydroxide because of the formation of soluble silver—ammonia complexes. A number of selective spot tests (24) iaclude reactions with /)-dimethy1amino-henz1idenerhodanine, ceric ammonium nitrate, or bromopyrogaHol red [16574-43-9]. Silver is detected by x-ray fluorescence and arc-emission spectrometry. Two sensitive arc-emission lines for silver occur at 328.1 and 338.3 nm. 

MetaUic impurities in beryUium metal were formerly determined by d-c arc emission spectrography, foUowing dissolution of the sample in sulfuric acid and calcination to the oxide (16) and this technique is stUl used to determine less common trace elements in nuclear-grade beryUium. However, the common metallic impurities are more conveniently and accurately determined by d-c plasma emission spectrometry, foUowing dissolution of the sample in a hydrochloric—nitric—hydrofluoric acid mixture. Thermal neutron activation analysis has been used to complement d-c plasma and d-c arc emission spectrometry in the analysis of nuclear-grade beryUium. 

Eq. (72) also shows that the intensity ratio of the atom and ion lines of an element will change considerably with the electron pressure in the plasma. Elements with a low ionization energy such as Na will thus have a strong influence on the intensity ratios of the atom and ion lines of other elements. This is analytically very important as it is the cause of the so-called ionization interferences, found in classical dc arc emission spectrometry but also in atomic absorption and plasma optical emission as well as in mass spectrometry. 

Table 7.8 Detection Limits for Elements in High Purity Copper Using DC Arc Emission Spectrometry...

Describe and illustrate a graphite electrode used for analysis of powdered samples by DC arc emission spectrometry. 

Metal and metal alloy producers and materials research laboratories routinely use spark and arc emission spectrometry and GD spectrometry has been gaining in popularity because it can analyze nonconducting solids directly. It is difficult to make direct sensitivity and accuracy comparisons between AAS, ICP, ICP-MS, and the arc/spark/glow discharge methods, since the latter techniques have sensitivities based on solid samples (LODs in p-g/g) and the former are solution techniques (LODs in pg/mL). While the solution detection hmits of graphite furnace or ICP may appear to be better than arc/spark or GD, the actual dilution factor for a real solid sample needs to be considered to compare the techniques for analysis of solids. 

The contribution of interference elements can be estimated by performing spectral line interference corrections. Calibrators are prepared in which mutually interferent elements are not present in the same solution. These solutions are then used to calibrate the system. Apparent concentrations are obtained by analyzing the ultrapure single element solutions (or solids). The interference coefficients are calculated by dividing the apparent concentration by the concentration of the interferent. In ICP-AES, the corrections are generally linear and thus a single element solution suffices to determine the correction factor. In spark and DC arc emission spectrometry, several samples are required. In practice, the determination of an element may be influenced by several other sample concomitants, and the final corrected concentration must be the summation of all the in-terferents. To complicate matters further, an iterative procedure must be used to deal with mutual interferences. 

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